SD, or ifSD?

Hi shooters. I'm new at the cerebral aspect of shooting and hunting (more of a shooter so SD never really entered my mind). I see the definition and I think about one of the weapons I own, and I just wonder if I truly can apply the math and conclude that the setup I'm using is any better than a factory load when it comes to penetration.

Leaving details out that are unimportant (so as to not seem self-serving more than anything.. I'm not here to say I have better stuff than anyone) when calculating SD, I do think that rather than using the diameter of the bullet you seated, you really need to understand that the bore diameter.. more specifically the diameter of the bullet as it LEAVES the barrel is the diameter that should be used to calculate SD when truly assessing/comparing theoretical performance.

For example, the various flavours of Lee Enfield rifles that were chambered for the 303=British commonly vary quite a bit in their measured bore diameters. If you fire a 311 diam bullet through a tight barrel, besides the horrible things going on inside there, the bullet actually exits with a higher SD than it would if it were fired through a worn or loose barrel.

So a question- if I buy 7.62 54r and pull the 180 gr 311 diam bullet out, neck-down the case and press in a 308 180gr bullet.. and fire it, does it exit with a SD that is any better than the factory 311 bullet?

My question stems from the claim one person made that making proper rounds for a tight bore will result in "better ballistics" due to the fact that it is smaller diam. I would agree "better ballistics" due to the loss of unnecessary friction and thus higher MV, but I'm here to argue that the diameter ends up being exactly the same and since the weight is the same the SD's are equal.

Hi shooters. I'm new at the cerebral aspect of shooting and hunting (more of a shooter so SD never really entered my mind). I see the definition and I think about one of the weapons I own, and I just wonder if I truly can apply the math and conclude that the setup I'm using is any better than a factory load when it comes to penetration.

Leaving details out that are unimportant (so as to not seem self-serving more than anything.. I'm not here to say I have better stuff than anyone) when calculating SD, I do think that rather than using the diameter of the bullet you seated, you really need to understand that the bore diameter.. more specifically the diameter of the bullet as it LEAVES the barrel is the diameter that should be used to calculate SD when truly assessing/comparing theoretical performance.

For example, the various flavours of Lee Enfield rifles that were chambered for the 303=British commonly vary quite a bit in their measured bore diameters. If you fire a 311 diam bullet through a tight barrel, besides the horrible things going on inside there, the bullet actually exits with a higher SD than it would if it were fired through a worn or loose barrel.

So a question- if I buy 7.62 54r and pull the 180 gr 311 diam bullet out, neck-down the case and press in a 308 180gr bullet.. and fire it, does it exit with a SD that is any better than the factory 311 bullet?

My question stems from the claim one person made that making proper rounds for a tight bore will result in "better ballistics" due to the fact that it is smaller diam. I would agree "better ballistics" due to the loss of unnecessary friction and thus higher MV, but I'm here to argue that the diameter ends up being exactly the same and since the weight is the same the SD's are equal.

There is a formula for BCs (G1 and G7) that works best for calculating Ballistic Coefficient based on bullet shape and type.

SDs are based on velocity averages and extreme spread.

Bryan Litz has a great book on Applied ballistics that will help you understand external Ballistics
better and will answer most if not all questions. (Not that I understand all there is to it, but it has helped me a great deal).

There is a formula for BCs (G1 and G7) that works best for calculating Ballistic Coefficient based on bullet shape and type.

SDs are based on velocity averages and extreme spread.

Bryan Litz has a great book on Applied ballistics that will help you understand external Ballistics
better and will answer most if not all questions. (Not that I understand all there is to it, but it has helped me a great deal).

J E CUSTOM

SD also means Sectional density... I think the OP is WAY over thinking it. SD is important but not THAT important.

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I'm 18"Firearms are second only to the Constitution in importance; they are the peoples' liberty's teeth."~George Washington

"The only advantage a light rifle has is weight, all other advantages go to the heavy rifle."
~ JE Custom

Location: The rifle range, or archery range or behind the computer in Alaska

Posts: 3,807

Re: SD, or ifSD?

Sectional density (as we use it in this context) is factored by two things. 1: Bullet weight 2: Bullet diameter. What ever the bullet weight/diameter is when it is in your hands may be slightly different when it exits the muzzle based on barrel dimensions and how bad it rakes copper off the jacket but you are splitting frog hair at this point. Smaller diameter bullets of equal weights of larger diameter bullets have higher sectional densities. The higher the sectional density, the more potential you have for a better BC. At least assuming you also have a good form factor. When it comes to equal weight + equal form factor, the higher the SD, the better the BC.

Shooting a 155 grain palma bullet in a Palma tight bore is not going to increase its sectional density and subsequent ballistic coefficient to any appreciable degree.

In theory, a 155 grain Palma bullet could change it by roughly 5% if it started out as a .308" diameter AND ended up exiting with a .300" dimension. That said, I'm no expert and I think the old Palma barrels (tight bore) were only .306ish. .300 might be a suicide mission.

Welcome to the forum.

M

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Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.

Hi shooters. I'm new at the cerebral aspect of shooting and hunting (more of a shooter so SD never really entered my mind). I see the definition and I think about one of the weapons I own, and I just wonder if I truly can apply the math and conclude that the setup I'm using is any better than a factory load when it comes to penetration.

Leaving details out that are unimportant (so as to not seem self-serving more than anything.. I'm not here to say I have better stuff than anyone) when calculating SD, I do think that rather than using the diameter of the bullet you seated, you really need to understand that the bore diameter.. more specifically the diameter of the bullet as it LEAVES the barrel is the diameter that should be used to calculate SD when truly assessing/comparing theoretical performance.

For example, the various flavours of Lee Enfield rifles that were chambered for the 303=British commonly vary quite a bit in their measured bore diameters. If you fire a 311 diam bullet through a tight barrel, besides the horrible things going on inside there, the bullet actually exits with a higher SD than it would if it were fired through a worn or loose barrel.

So a question- if I buy 7.62 54r and pull the 180 gr 311 diam bullet out, neck-down the case and press in a 308 180gr bullet.. and fire it, does it exit with a SD that is any better than the factory 311 bullet?

My question stems from the claim one person made that making proper rounds for a tight bore will result in "better ballistics" due to the fact that it is smaller diam. I would agree "better ballistics" due to the loss of unnecessary friction and thus higher MV, but I'm here to argue that the diameter ends up being exactly the same and since the weight is the same the SD's are equal.

Am I off track here? or just thinking too much?

After reading your post again To make sure I understood your question I have more comments.

First- if you ask a question don't get offended, everyone has there opinion and it may be different
than yours or someone else. (It is just a discussion).

With that said, There are two diameters in a barrel, There is the bore diameter(Grove diameter)
and the land diameter (Rifling) Bore diameter is the same (Or Close) to the bullet diameter.

Land diameter is less than the bore diameter so it can engrave the bullet. This diameter may vary based on the barrel makers preference and number of rifling's/lands (2,3,5,6 and 8). they may vary
from .003 to .005 In height. Example: A 308 win would have a bore diameter of .308 and a land
diameter of .300 (.004 land height) +/- .0001 or .0002.

And yes, accuracy if at its best with a good fitting bullet but the ballistic coefficient is not changed
much by the rifling process. Sectional density is changed slightly by the bullet being compressed
in the engraving process but again ,not much.

In general the ballistic coefficient is set by the bullet maker based on the length, weight and diameter.
Shape can alter the length of a bullet of the same weight and caliber of another bullet.

There are lots of things that have to be right to have a good shooting rifle and in the long range
game everything comes into play. High ballistic coefficient's, low Standard deviations, powder densities
reloading prowess, the best barrels, perfect chambers and head space, and more.

Then add the shooters ability.

Being aware of the quality of your system is paramount to long range hunting /shooting but there
is a point of diminishing returns and some things are not worth the effort and everyone has there
own idea where that is.